Xerographic development electrode apparatus

ABSTRACT

Improved xerographic development electrode apparatus for eliminating the problem of clogging of developer composition in the development zone associated with high speed development. The development electrode comprises a plurality of taut parallel spaced elongated wire electrodes supported by a housing element.

O Umted States Patent 11 1 1111 3,778,144 Hudson [4 Dec. 11, 1973 [54] XEROGRAPl-HC DEVELOPMENT 3,011,474 12/1961 Ulrich 118/637 C OD APPARATUS 3,147,147 9/1964 Carlson 118/637 3,393,663 7/1968 Donalies 118/637 [75] Inventor: Frederick W- Huds n, s 3,472,657 10/1969 Mayer ct al 355 3 x Henrietta, NY.

[73] Assignee: Xerox Corporation, Stamford, Primary Examiner-Samuel S. Matthews Conn. Assistant ExaminerKenneth C. Hutchison 1 Filed g 18 1971 AttorneyJames J. Ralabate et al.

[211 Appl. No.: 172,839 [57] ABSTRACT Improved xerographic development electrode appara- 355/3 gd i ggg tus for eliminating the Problem of clogging of develp composition in the development Zone associated [581 Fleld of Search 355/3, 17, 118/637 with g Speed development. The development elec [56] References Cited trode comprises a plurality of taut parallel spaced elongated wire electrodes supported by a housing ele- UNITED STATES PATENTS ment. I 3,606,863 9/1971 Latone 118/637 3,331,355 7/1967 Donalies et al. 118/637 4 Claims, 3 Drawing Figures PAIENTEU INVENTOR FREDERICK W. HUDSON TTORA/E Y XEROGRAPIIIC'DEVELOPMENT ELECTRODE APPARATUS BACKGROUND OF'THE INVENTION This inventionrelates to a development electrode apparatus for developing electrostatic latent images formed on a xerographic plate such as a xerographic photoconductive .belt assembly.

In the process .of xerography, as disclosed in Carlson U.S. Pat. No. 2,297,691, axerographic' plate comprising a layer of photoconductive insulating material on a conductive backing is given auniform electric charge over its surface and is then exposed to the subject matter to be reproduced, uusually by conventional projection .techniques. This exposure discharges the plate areas in accordance withthe radiation'intensity that reaches them and thereby creates. an electrostatic la- 1 tent image on 'or in the photoconductive layer. Development of the. latent image is effected with an electrostatically charged, finely divided material such as an electroscopic powder that is brought into surface contact with the'photoconductive layer 'and is held thereon electrostatically in a pattern corresponding to xthe electrostatic .latent image. Thereafter, the. devel-.

oped xerographic powder image is usuallytransferred to a support surface to which it may be fixed by any suitable means.

Many development systems forv developing electrostatic latentimages are known in the xerographic arts 'including brush, magnetic brush, liquid'and cascade, to 'name a few. The most commonly employed of the above that enjoys by'far the widest commercial utility is the cascade system in which a two-component developer composition, as for example disclosed in U.S. Pat. No. 2,6l8,552, is caused to cascade over the charge bearing surface of the xero'graphic-plate to effect development of the image thereon. The two-components of the cascade developer include a relatively hardcarrier the electrostaticlatent image withsh'arpness and uniform densitypl'lowever', where'the copy to be reproduced is possessed of'images having at least some areas of other than line copy form,such as solidly wide, dark areas termed solid area," the cascade development technique is by itself known to 'be inherently unable to effect faithful development thereof. Instead of a faithful, uniform deposition acrossthe entireimage'area, there results a halo appearing development consisting of developed fringes about the image perimeter and an absence 'of'developer in the-central portions. owing to the improper and non-uniform adherence of the developer powder thereat. This effect is believed caused by jthe characteristics of "the charged forces associated with the central, portions of the solid area. Rather than the electrostaticlines of force being directed outwardly perpendicular to the plate surface, they'are directed instead in curved paths to the edges or outer portions 'thereof into the adjacent discharged orbackground areas'. This renders the force lines of insufficient magnitude to attract the developer powder from the carrier and results in the central portions being vacant of developer.

It has been found that positioning an electrode in close proximity to the image bearing surface during development improves the continuity in development in low contrast image areas. The use of such electrodes is disclosed in U.S. Pat. Nos. 2,573,881 and 2,777,418. This phenomenon apparently results from the presence of the electrode which supplies a nearby potential gradient for setting up the electrostatic lines of force. The

use of such development. electrodes, however, has caused some complications in the developing process. For example, in order to be most effective, the development electrodes require close spacing to the image bearing surface. Therefore, when cascade development is used, the development material has a tendency to bunch up between the image bearing surface and the development electrode with consequent smudging of the image as well as scratching and abrasive deterioration of the image bearing surface. In addition, these development electrodeshave impeded the flow of the developing material to the image bearing surface thereby slowing the development process. Some of these problems can be overcome by cascading less material at a time but such a decrease in development material either requires a reduction of machine speed or produces aless densely developed image. Attempts to replace the solid plate development electrode with a .series of grid wires or a perforated plate have resulted in a loss of effectiveness in the electrode. An effective development electrode appears to require a large surface area, or a semblance thereof, in close proximity to the image bearing surface.

As mentioned above, a development electrode comprising a series of wires has been found to be ineffective which regard to the cascade development process. In U.S. Pat. No. 2,784,109 to Walkup, a development electrode comprising a series of fine, parallel grid wires stretched out between two sides of a rigid metal frame is disclosed. While the development electrode disclosed in this patent obviates the jamming problem encountered with cascade development, the wires are spaced at such intervals that the field effect desired from the wires (i.e. the solid area enhancement) decreases. In yet another development apparatus disclosed in U.S. Pat. No. 3,349,676 to Hudson, a device is disclosed which comprises a plurality of flexible,

. elongated hanging electrode chains which drag on the 'photoconductivesurface. While this particular type of development'electrode assembly alleviates the problem of jamming in the process of cascade development, the

requisite spacing of'the elongated electrode chains causes a dilution in the-electrode effect thereby resulting in pockets of undeveloped areas in the solid areas to be reproduced. In both patented devices, if the wires or elongated chains are multiplied, the cascade development process is impeded due to the clogging of the developer composition.

SUMMARY OF THE INVENTION It is a further object of the present invention to improve the construction and arrangement of a development electrode means in a xerographic reproducing ap paratus which is free from excess accumulation of developer material.

It is yet a further object of the present invention to provide a development electrode arrangement for use in a xerographic reproducing apparatus which provides maximum efficiency with regard to flow of developer in a cascade development system and which provides maximum field effect required for solid enhancement of copy.

These and other objects of the present invention are attained in the development electrode apparatus as contemplated by the present invention which comprises a series of fine, parallel wires tautly stretched between two sides of a rigid rectangular structure, the wires being of a thickness and being spaced such that there is no clogging of developer during cascade development and yet retaining complete continuity in the electrode effect of the device. It has been found that the wire development electrode of the present invention is especially useful for high speed development.

Other objects of the invention will become readily apparent to those skilled in the art in view of the following detailed disclosure and description thereof, especially when read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS DESCRIPTION OF A PREFERRED EMBODIMENT For a general understanding of the xerographic processing system in which the invention is incorporated, reference is made to FIG. 1 in which the various system components are schematically illustrated. As in all xerographic systems based on the concept disclosed in the aforementioned Carlson patent, a radiation light image of a copy to be reproduced is projected onto the sensitized surface of a xerographic plate to form an electrostatic latent image thereon. Thereafter, the latent image is usually developed with an oppositely charged developing material to form a xerographic powder image, corresponding to the latent image on the plate surface. The powder image is then electrostatically transferred to a support surface to which it may be fused by any suitable form of fusing device, whereby the powder image is caused permanently to adhere to the support surface.

In the apparatus illustrated, there is shown a xerographic copier, having a photosensitive element 60 on which a latent electrostaticimage of the information to be reproduced is formed. The photosensitive element is in the shape of a belt which is slidably mounted and driven by shafts 62. The photosensitive belt comprises an electrically conductive substrate material 63 which, in turn, is covered on its other surface with a layer 64 of a photoconductive insulating material such as-vitreous selenium. The photosensitive belt element has five processing stations located about its periphery which carry out the basic steps of the xerographic process. These stations include charging station 10', exposing station 20, developing station 30, transfer station 40, and cleaning station 50.

The latent electrostatic image is formed on the photosensitive belt surface by passing it through charging station 10 and exposing station 20. The charging station consists of any suitable means for placing a uniform charge on layer 64 such as corona charging device 11 shown in the drawing. Exposing station 20 comprises projector 21 which projects and focuses a light pattern on the photosensitive surface conforming to the image to be reproduced by the xerographic system. The image projected onto the photosensitive layer of the belt causes selective charge dissipation on elemental areas of layer 64 to form a latent electrostatic image thereon. Other means for forming electrostatic images on insulating surfaces are known in the art and may be used instead of the one shown.

After the formation of the latent electrostatic image by passing the photosensitive belt through stations 10 and 20, the photosensitive element is then forwarded to developing station 30. At this point, a developer mixture comprising a toner powder composition and carrier material is cascaded over the belt surface whereby the toner particles are attracted to and deposited on the charged portions of the surface conforming to an image configuration. The toner particles used generally consist of resinous electroscopic powder while the carrier material is a coarse, granular material generally re ferred to as carrier beads. The development phenomenon occurs because of the fact that when the toner and carrier beads are mixed, toner particles are attracted to the large carrier bead nucleus through triboelectric attraction. Consequently, when the developer mixture is cascaded over the surface of the photosensitive belt the toner particles are attracted from their respective carrier beads and deposited on the photosensitive surface by the electrostatic charge pattern thereon.

The cascade development assembly comprises housing 31 containing a quantity of developer composition. The mixture is lifted to an elevate'dposition over the photosensitive surface by buckets 34 which are supported on conveyor 33 which, in turn, is supported and driven by pulleys 35. The development mixture lifted to the elevated position over the photosensitive surface is dumped onto the surface in the vicinity of the deflector plate 37. The deflector plate directs the developer mixture onto the surface of the belt where it flows across said surface through development zone 39 and then into the trough 38 of housing 31. The development zone 39 contains the development electrode of the present invention which is shown in greater detail in FIG. 2 and is spaced from the surface of the belt. Development electrode 70 contains a plurality of taut, parallel spaced elongated wires 72 within housing 71 which enable the developer composition to flow freely within the gap between the development electrode and the photosensitive belt when the electrode is alligned in the direction of developer flow. During the development process, the deflection plate 37 developer composition cascades onto the and from there flows between the photosensitive belt surface and the development electrode whereupon the toner is picked up on the charged areas. Because of the parallel spaced taut wires of the electrode and their alignment generally parallel to the direction of developer flow, the developer composition is allowed to move continuously uninhibited along the extended planar surface of the photosensitive belt whereupon full development of the image bearing surface takes place. The lower end 74 of the developer electrode extends beyond the lower end of the planar surface of the photosensitive belt whereby the excessive developer'can freely flow (as indicated by reference numeral 76) back into the trough 36 whereupon denuded carrier beads pick up more toner which is recycled again through the cascade development process.

Following the development step, the belt carries the developed image through transfer station 40 where the toner image is transferred from the photosensitive surface to support material 41. Transfer of the developed image onto the support material is aided by a corona device 42 which applies an electrostatic charge having a polarity opposite that of a triboelectric charge on the toner particles to the support material. Transfer takes place between guide rollers 45 which act to position the support material against the photosensitive belt at the transfer station. Support material 41 is fed from reel 43 before transfer and is rewound on reel 44 after transfer takes place. If the support material is the permanent substrate upon which the developed image is to be fixed, fusing device 46 is desirable and should be placed along the path of this support material between the point where the developed image is transferred to the support material and the point where the support material is rewound on reel 44. The fusing device is positioned to heat the toner particles thereby permanently fixing them to the support material.

The final station shown in the drawing is cleaning station 50 comprising a cylindrical fur brush 51 which contacts the photosensitive surface of the belt. The cleaning station is utilized to remove any residual toner particles from the photosensitive surface after transfer occurs and before a new image cycle is begun. It is intended that various moving elements be driven by any suitable means, for instance, the motor that drives the belt 63 can also, through appropriate gearing, drive all the other moving elements in the copier.

In FIG. 2, there appears a frontal view of the development electrode of the present invention. As can be seen, a plurality of spaced wires 72 are fixed tautly to housing 71. As mentioned above, in one embodiment of the present invention, the development electrode is placed in conjunction with the cascade development system such that the wires are substantially parallel to the direction of movement of the xerographic plate and also to the direction of developer flow. In addition, as heretofore mentioned, the diameter of the wire as well as the distance between wires are critical elements in obtaining optimum results with respect to the instant development electrode. Should the wires be spaced too closely together, then jamming of the developer composition under-the wires will occur. Should the wires be spaced too far apart then there will be a discontinuity in the field effect.- Within the purview of the present invention, the diameter of thewire should be in the range of 0.004 to 0.050 inches and spaced at distances of from about 0.010 to 0.250 inches center to center spacing. Within these defined parameters it is found that the development electrode enables the developer composition to flow freely over the photosensitive belt surface with no dilution of the field effect of the electrode. Optimum results are obtained with an electrode having 0.010 inch wire with 0.045 inch center to center spacing.

Reference to FIG. 3 demonstrates another embodiment of the present invention. As can be seen, the development electrode is inclined at an angle to the direction of travel of the photoconductive belt surface 60. The angle of the skewed development electrode has been accentuated in the drawing for purposes of illustration but in actual practice an angle of only a few degrees from the direction of belt travel is required to effeet the beneficial results of this particular embodiment. In actual operation, the developer composition 36 flows in a direction counter to the direction of the photosensitive belt and encounters the development electrode. Because of the angle of the development electrode, accumulated toner is forced to flow across the belt surface in a manner such that total development necessarily takes place. The slight angle of the development electrode not only insures complete development but also obviates the possibility of line gaps in the ultimate copy stemming from the total alignment of the taut parallel spaced wires in the direction of flow.

It is preferable in practicing this invention that the development electrode be made of a suitable conducting material in that a sufficient number of parallel placed lines of such material be present to insure a continuous field. The conducting material may consist of aluminum, stainless steel, copper, nickel and compositions thereof.

While the invention has been described with reference to preferred arrangements and embodiments, it will be generally understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the true spirit and scope of the invention.

I claim:

1. In a xerographic reproduction unit including a photosensitive surface; means to form an electrostatic latent image on said surface and a developing apparatus for delivering two-component cascade developer to said surface at a developing station for cascade development of the latent image thereon;

a development electrode apparatus comprising a plurality of taut generally parallel spaced conductive members located in said development station and in a plane spaced from and generally parallel to said photosensitive surface;

said conductive members extending in the same general direction as the movement of the cascading developer;

said conductive members each having a maximum cross sectional dimension of from about 0.004 to 0.050 inch and center to center spaced from each other from about 0.010 to 0.250 inch;

said photosensitive surface extending away from and at an angle to said plane of said conductive members at the lower end of the development station;

said conductive members extending substantially beyond the lower end of the development station to the extent that excessive developer freely flows off said photosensitive surface;

and means to contact said electrodes to a reference potential.

wherein the maximum cross sectional dimension of the conductive members is each no greater than about 0.010 inch and the conductive members are spaced from each other at a distance of no greater than about 0.045 inch. 

1. In a xerographic reproduction unit inCluding a photosensitive surface; means to form an electrostatic latent image on said surface and a developing apparatus for delivering two-component cascade developer to said surface at a developing station for cascade development of the latent image thereon; a development electrode apparatus comprising a plurality of taut generally parallel spaced conductive members located in said development station and in a plane spaced from and generally parallel to said photosensitive surface; said conductive members extending in the same general direction as the movement of the cascading developer; said conductive members each having a maximum cross sectional dimension of from about 0.004 to 0.050 inch and center to center spaced from each other from about 0.010 to 0.250 inch; said photosensitive surface extending away from and at an angle to said plane of said conductive members at the lower end of the development station; said conductive members extending substantially beyond the lower end of the development station to the extent that excessive developer freely flows off said photosensitive surface; and means to contact said electrodes to a reference potential.
 2. The xerographic reproduction unit of claim 1 wherein the conductive members extend in a direction parallel to the direction of developer flow.
 3. The xerographic reproduction unit of claim 1 wherein the conductive members extend in a direction at a slight angle to the direction of developer flow.
 4. The xerographic reproduction unit of claim 1 wherein the maximum cross sectional dimension of the conductive members is each no greater than about 0.010 inch and the conductive members are spaced from each other at a distance of no greater than about 0.045 inch. 